While the development of effective factor VIII concentrates has markedly improved our ability to treat hemophilia and to prevent many of its serious complications, the development of antibodies that inactivate factor VIII (antihemophilic factor) -- "factor VIII inhibitors" -- is a persisting severe complication that is responsible for a large proportion of patient disability and hemophilia care cost. This research program seeks information about the nature of factor VIII inhibitors and the antigenic determinants with which they react. It is based on the contention that effective treatment and/or prevention of factor VIII inhibitors is not likely until this basic information is available. Recent studies have established that a subset of factor VIII inhibitors, those that bind to factor VIII light chain epitopes, inactivate factor VIII by preventing its effective interaction with phospholipid. The proposed studies will further characterize the molecular basis of factor VIII inhibitor function. This will include an examination of the possible impact of factor VIII inhibitors on the cleavage of factor VIII by thrombin and on factor VIII interaction with factor IXa and with factor X. The critical issue is definition of inhibitor effect and the use of this information to better understand factor VIII structure/function properties. The antigenic properties of factor VIII will also be characterized in more detail, using methods that can identify determinants that may not be detected by standard immunoblotting protocols. As infusion of large amounts of factor VIII on a regular basis has led to reduction in the anti-factor VIII titer and an apparent "tolerant" state in some patients, it is important to understand the mechanism by which this effect is produced. Characterization of antibody development during tolerance induction protocols will address the hypothesis that a "blocking antibody" is formed. Finally, the molecular defects responsible for CRM-positive and CRM-reduced hemophilia A will be determined using immunoaffinity purification and immunoassay techniques. While the emphasis will be on characterization of the nonfunctional molecules in patient plasma, parallel gene studies will be carried out to establish molecular genetic basis of the disorders.